The conventional electrophotographic process has an inherently lower gain than the silver halide photographic process. A low exposure in an electrophotographic process results in a small amplitude differential voltage pattern on a photoconductor, and when developed with conventional toner, the resulting image is of low density. It has been a longstanding goal to increase the gain of the electrophotographic process so that higher density images may be produced from low exposures. This is of particular concern in applications such as diagnostic xeroradiography, where the exposing x-rays pose a potential health threat to the patient, and the lowest exposure possible is desired. It has also been a goal to produce an electrical image signal from an x-ray image so that the image signal can be electronically processed to enhance the visibility of certain features, stored in an electronic form, or transmitted electronically to a remote location.
Increases in gain or photographic speed are also desirable in other electrographic processes. As , used here, electrographic processes are processes in which differential patterns of voltage, charge, current or conductivity are developed with toner. Electrographic processes include, for example, dielectric recording, photoconductography, electrophotography, photoelectrophoresis, ionography,
It has been proposed in U.S. Pat. No. 4,624,543 issued Nov. 25, 1986 to Young et al. to generate an electrical image signal by exposing a photoconductor to produce a differential voltage pattern on the photoconductor, developing the image on the photoconductor with a conventional liquid toner employing a development electrode, scanning the developed image with a beam of light, and detecting the modulation of the light beam by the developed image. Light that strikes the untoned portion of the image is specularly reflected from the surface of the photoconductor. Light that strikes the toner is diffusely reflected. The detector optics are arranged to detect only the specularly reflected portion of the light.
If the apparatus described in U.S. Pat. No. 4,624,543 is employed to generate an electrical image signal from a small exposure (for example a small x-ray exposure), the small exposure results in a toned image that is of very low density. Since the photodetector senses the modulation by the toner image of the specularly reflected light of the to detect for a low density image. The resulting electrical image signal is a complicated function of the optical properties of both toner and substrate.
Also, in the prior art method, the presence of a small coverage of toner is detected as the difference in two large signals: the reflectance signal from a clean photoconductor, and the slightly different reflectance signal from a photoconductor with a small coverage of toner. The resulting difference signal will be noisy. Further, the method is sensitive to slight scratches or imperfections on the surface of the photoconductor. Finally, the use of a Fresnel type lens to collect the specular light over a large scanning area, as taught in col 6, pp 47-55 of U.S. Pat. No. 4,624,543, will introduce unwanted ripples in the output signal which may interfere with the detection of small modulations in the toner coverage, thereby making the signal difficult to interpret.
Another limitation of the above process is the restriction to reflective substrates.
Another prior art method of generating an electrical image signal from an x-ray exposure is the storage phosphor technique described in U.S. Pat. No. Re. 31,847, reissued Mar. 12, 1985 in the name of George W. Luckey. In the storage phosphor approach a stimulable storage phosphor is exposed to a pattern of radiation to form a stored latent image. The stored image is read out by stimulating light, which releases a pattern of radiation that is detected to generate the image signal. This approach suffers process, in which the latent image is destroyed when is read out. Another drawback is that the stimulating exposure needed is relatively high, requiring expensive readout apparatus. Yet another drawback is the loss of resolution caused by scattering of the stimulating light in the stimulable phosphor.
It is therefore the primary object of the an electrical image signal from a low exposure in an electrophotographic process that is free from the shortcomings noted above. It is a further object of the invention to provide means of detecting very low coverages of toner produced image-wise by other electrographic methods such as ionography, stylus
of voltage, charge, current or conductivity is toned.